BOT 161 Notes Syllabus Theme B B1: NAMING AND CLASSIFICATION OF ORGANISMS

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1 BOT 161 Notes Syllabus Theme B (Copyright reserved) Dr TE Tshikalange Natural Sciences II Building Room 3-30 Tel: Very important: These notes are only a summary and not complete. The prescribed pages in Raven et al. (2005) must be used when preparing for tests and exams. B1: NAMING AND CLASSIFICATION OF ORGANISMS Taxonomy is the science of identifying, naming and classifying organisms Classification: Grouping organisms by their similarities or relationships Systematics: Analytical study of the diversity and relationships of organisms Why we use Latin names to name living organisms? Historical development Theophrastus 4 th Century 500 plants on basis of leaf characteristics 13 th Century Monocot/Dicots based on stem structure 18 th Century flower and fruit structures used in classification Latin used widely in schools and univ. customary to use it Mentha (genus name) Mentha floribus spicatis, foliis oblongis serrattis (phrase name) Binomial system (by Carolus Linnaeus Swedish ) Organism are named according to the binomial system of nomenclature, that was started by Linnaeus. Based on a set of rules: - name consists of 2 parts (binomial): a generic name + a specific epithet Phaseolus vulgaris L. Phaseolus vulgaris L. 1

2 Rules and principles for the granting of a name It is according to the International Code for Botanical Nomenclature (I.C.B.N.) and is based on the following 6 principles: a. plant nomenclature is independant of zoological nomenclature; b. the rules of the code are retrospective; c. the names are published in Latin, irrespective of its origin; d. the nomenclature of any taxon is based on the priority of publishing; e. every taxon can have only one correct name & it is the oldest name according to the rules; f. the application of botanical names is determined by means of nomenclatural types (typification). The main aim of the code is to ensure that each taxon has only one name and that a taxon s name is correct Taxonomic ranking a. Taxon is a group of organisms (irrespective of its size) of which all members share certain common characteristics. b. It starts with the individual (species lowest rank), leads to higher groups and ends with the kingdom (highest rank). c. The level of a category in the hierarchy is its rank. d. The aim of a taxonomic ranking (hierarchy) is to arrange the taxa in such a way to reflect at best the differences and similarities amongst them. Main ranks in a hierarchigal system Closely related species sharing communal characteristics are assigned to the same genus, and closely related genera are grouped in a single family. Families are grouped into orders, orders into class, classes into phyla, and phyla into kingdoms (regnum). Any group of organisms whether it be a genus or a kingdom, is referred to as a taxon, and the status of the taxon is referred to as the category. A kingdom (regnum) is the highest taxon used in the classification of living organisms. 2

3 CLASSIFICATION OF MAIZE (Zea mays) CATEGORY NAME OF TAXON DESCRIPTION Kingdom Plantae Multicellular eukaryotic organisms, photosynthetic Phylum Anthophyta Flowering plants with ovules born in a closed ovary Class Monocotyledones Embryo with one cotyledon, flowers trimerous Order Commelinales Reduced flower parts, elongated leaves and dry 1-seeded fruits Family Poaceae Grass family. Reduced green flowers in spikelet. Fruit is a caryopsis (grain) Genus Zea Tall annual grass with separate female and male flowers Species Zea mays Only one species in genus - maize 3

4 B2: SYSTEMATICS Phylogeny: Evolutionary history of a group of organisms from a common ancestor Classifications based on common ancestors Traditionally based on structural similarities Classification systems: Artificial: Theophrastus classified according to plant growth forms Linnaeus classified according to the sexual system (nr. of stamens) - Usually concerns only one or a few characters Natural: Classifications to be an accurate reflection of the evolutionary relationships among organisms - Based on as many features as possible Taxa should reflect evolutionary relationships: - If all of the members of a taxa share a common ancestor, it is monophyletic and reflects true evolutionary relationships - A taxon containing the ancestor and all organisms derived from it is called a clade - Paraphyletic taxon consists of a common ancestor and some, but not all, of its descendants. - Many taxa are polyphyletic, evolved from different ancestors. Homology vs Analogy: Homologous structures that have a common origin but not a common function Eg. Foliage leaves, cotyledons, bud scales and floral parts WHY? Analogous structures that have a similar function and appearance but have an entirely evolutionary background Eg. Spine of a cactus and a stem thorn WHY? Cladistics: Cladistics emphasizes phylogenetic relationships. - Cladists use shared derived characters to reconstruct phylogenies by outgroup analysis - Cladograms illustrate branching between taxa - Branching points are called nodes 4

5 - Ancestral and derived characters are used to construct cladograms Fig page 225 Cladistics is the newer, but currently most employed tool in modern taxonomy - Critical to cladistic analysis is outgroup analysis, guided by parsimony - Groups that share similar evolved traits, called derived characters, are considered to be closely related - An outgroup is a taxon, which is considered to have diverged earlier than the ingroup taxa of inconsideration: - A classic outgroup is the closest relative of the taxon being studied - An ingroup shares many characters and is closer to the ancestral condition - Outgroup analysis is used in building and interpreting cladograms: - After selection of taxa, homologous characters are selected - Characters must be defined - Characters must be organized in evolutionary order - Groups are characterized as relative outgroups if they lack the ancestral condition, or ingroups if they possess the ancestral character - A cladogram is constructed by considering shared derived characteristics - Taxa are grouped by shared derived characters - The base of the cladogram represents the common ancestor for the taxa: being analyzed - Nodes represent branching point - The branching process is continued until all clades are represented - In a cladogram, each branch point represents a major evolutionary step: - Organisms connected at a node share a common ancestor at that point - By comparing the distance from the node to the ancestral organism, evolutionary distances cannot be measured exactly; it is a relative measure only - Cladograms show that taxa share a common ancestor, but that ancestor remains unspecified (note that there is not a specific organism named at the node, nor at the base of the cladogram) Molecular biology provides taxonomic tools: - Molecular systematics focuses on the use of molecular structures to clarify evolutionary relationships 5

6 - Technique: Determine the sequence of amino acids in proteins and nucleotides in nucleic acids. - Advantages: - Drawback: - easy to quantify - provide many more characters for phylogenetics - allow comparison of very different organisms - molecular data cannot be obtained from fossils etc. - Amino acid sequences may be used as molecular clocks - Measure time since divergence from a common ancestor by the number of differences in nucleotide or protein sequences - Rate of change must be constant - Studies of ribosomal RNA structure may indicate phylogenies - Mitochondrial DNA may also indicate relationships 6

7 B3: KINGDOM PLANTAE SIX-KINGDOM SYSTEM In the past, organisms were divided into two groups. They were thought to be either (1) "animals" that moved, ate things, and breathed, and the size of their bodies was definitely limited; or (2) "plants" that did not seem to move, eat, breathe and seemed to grow indefinitely. In the course of time, new organisms were discovered which were classified as being either plants, or animals. In this way the fungi and bacteria were classified as plants and the protozoa as animals. It became increasingly evident, however, that the division of all organisms into two groups was an over-simplification of the matter. Today, six kingdoms (regna) are suggested: two prokaryotic kingdoms (i.e. the genetic material consists of a single, circular DNA molecule not enclosed by a nuclear membrane) namely the Eubacteria and Archaea and four eukaryotic kingdoms (i.e. the genetic material consists of chromosomes enclosed by a nuclear membrane) namely the Protista, Fungi, Plantae and Animalia. Kingdom Eubacteria Prokaryotic organisms lack distinct nuclei and other membranous organelles. Unicellular, microscopic, cell walls generally composed of peptidoglycan. Most are decomposers; some parasitic (and pathogenic); some chemosynthetic autotrophs; some photosynthetic; important in recycling nitrogen and other elements; some used in industrial processes. Kingdom Archaea Prokaryotic organisms - lack distinct nuclei and other membranous organelles. Unicellular, microscopic, peptidoglycan absent in cell walls; differ biochemically from eubacteria. Methanogens are anaerobes that inhabit sewage, swamps and animal digestive tracts; extreme halophiles inhabit salty environments; extreme thermophiles inhabit hot environments. Kingdom Protista Eukaryotic organisms which are unicellular or simple multicellular. Three informal groups include protozoa, algae and slime and water moulds. Feeding mechanisms: photosynthesis, absorption, ingestion, or a combination of these. Algae are important producers especially in freshwater and marine systems. Kingdom Fungi Eukaryotic organisms with complicated life cycles. Body composed of threadlike hyphae that form tangled masses that infiltrate food or habitats. Cell walls have chitin. Feeding mechanism: heterotrophic, they absorb nutrients from living or dead organisms and do not photosynthesise. Decomposers, some parasitic (and pathogenic); responsible for much spoilage and crop loss; some used in food and alchoholic beverages; some used to make antibiotics 7

8 Kingdom Animalia Multicellular, eukaryotic organisms without plastids and photosynthetic pigments. Exhibit tissue differentiation and complex organ systems. Mobile. Feeding mechanism: mostly intake. Consumers herbivores, carnivores or detritus feeders. Kingdom Plantae Multicellular, eukaryotic organisms with plastids. Photosynthetic. Alternation of generations (diploid and haploid). Cell walls have cellulose. Most have roots, stems and leaves. Feeding mechanism: autotrophic. Includes mosses, ferns and vascular plants. FOUR MAJOR GROUPS OF PLANTS Bryophytes (mosses) - lack specialised vascular tissues - reproduce via spores Seedless vascular plants (ferns) - reproduce via spores Seed plants: Gymnosperms (conifers, cycads) - produce seeds that are exposed (cones) Angiosperms (flowering plants) - produce seeds enclosed in a fruit ANGIOSPERMS The most diverse and successful group of plants Vascular plants that produce flowers and seeds enclosed within a fruit Flowers may contain sepals, petals, stamens, carpels Ovules are enclosed within an ovary After fertilization, ovules become seeds and the ovary develops into a fruit Life cycle: Sporophyte generation is dominant Gametophytes are - Reduced in size - Nutritionally dependent on sporophyte generation 8

9 Heterosporous Within the flower, they produce microspores and megaspores - Microspore develops into a pollen grain - Megaspore develops into an embryo sac Double fertilization resulting in formation of a diploid zygote and a triploid endosperm Phylum Anthophyta is divided into two classes: Monocots Floral parts in multiples of three Seeds that each contain one cotyledon Nutritive tissue in their mature seeds is endosperm Dicots Floral parts in multiples of four or five Seeds that each contain two cotyledons Nutritive organs in their mature seeds are the cotyledons 9